Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide a system and method for regulating power conversion systems. Merely by way of example, some embodiments of the invention have been applied to power conversion systems operating in a quasi-resonant mode. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 108, a secondary winding 110, an auxiliary winding 112, a power switch 106, a current sensing resistor 104, two diodes 114 and 116, capacitors 118, 120 and 126, a rectifying bridge 128, resistors 130, 132 and 134, a system controller 102, an AND gate 172, an OR gate 174, and an isolated feedback component 103. The isolated feedback component 103 includes resistors 136, 138, 140 and 142, capacitors 122, 124 and 146, a three-terminal regulator 143, and an opto-coupler 144. The system controller 102 includes a resistor 148, a comparator 150, a demagnetization detector 152, and a flip-flop component 154. For example, the power switch 106 includes a bipolar junction transistor. In another example, the power switch 106 includes a field effect transistor (e.g., a metal-oxide-semiconductor field effect transistor). In yet another example, the power switch 106 includes an insulated-gate bipolar transistor.
As shown in FIG. 1, the power conversion system 100 uses a transformer including the primary winding 108 and the secondary winding 110 to isolate a primary side and a secondary side of the power conversion system 100. Information related to an output voltage 156 on the secondary side can be extracted through a voltage divider including the resistors 138 and 142. A feedback signal 158 is generated based on information related to the output voltage 156. The controller 102 receives the feedback signal 158, and generates a drive signal 160 to turn on and off the switch 106 in order to regulate the output voltage 156.
If the power switch 106 is closed (e.g., on), the energy is stored in the transformer including the primary winding 108 and the secondary winding 110. Then, if the power switch 106 is open (e.g., off), the stored energy is released to an output terminal 166, and the system 100 enters a demagnetization process. A signal 198 (e.g., INV) maps a winding voltage 196 of the auxiliary winding 112 through a voltage divider including the resistors 132 and 134. The demagnetization detector 152 detects the demagnetization process using the signal 198, and outputs a detection signal 194 to the AND gate 172 which also receives a signal 173 associated with a maximum operating frequency of the system 100. The OR gate 174 receives a signal 175 from the AND gate 172 and a signal 176 associated with a minimum operating frequency of the system 100 and outputs a signal 178 to the flip-flop component 154 (e.g., at terminal S). The comparator 150 receives a current sensing signal 192 and the feedback signal 158 and outputs a comparison signal 190 to the flip-flop component 154 (e.g., at terminal R).
Upon the completion of the demagnetization process (e.g., the stored energy being completely released to the output terminal 166), series resonance occurs between the primary winding 108 and a parasitic capacitor 168 of the power switch 106. If a voltage drop of the capacitor 168 reaches a local minimum value (e.g., the voltage drop between the terminals of the power switch 106 reaching the local minimum value) during the series resonance, the system controller 102 changes the drive signal 160 to close (e.g., turn on) the power switch 106. The switching loss of the switch 106 is reduced and the efficiency of the power conversion system 100 is improved. For example, a switching period of the switch 106 includes an on-time period during which the switch 106 is closed (e.g., on) and an off-time period during which the switch 106 is open (e.g., off).
FIG. 2 is a simplified conventional timing diagram for the flyback power conversion system 100 that operates in the quasi-resonant (QR) mode. The waveform 202 represents the drive signal 160 as a function of time, the waveform 203 represents the feedback signal 158 as a function of time, the waveform 204 represents the current sensing signal 192 as a function of time, the waveform 206 represents the signal 198 as a function of time, and the waveform 208 represents the detection signal 194 as a function of time. For example, t0≦t1≦t2.
During a time period between t0 and t1, the drive signal 160 is at a logic high level (e.g., as shown by the waveform 202), and the power switch 106 is closed (e.g., on). An input voltage 186 applies on the primary winding 108, and a current 188 flows through the primary winding 108. The current sensing signal 192 increases in magnitude (e.g., as shown by the waveform 204). The signal 198 keeps at a low magnitude (e.g., 0), and the detection signal 194 keeps at a low magnitude (e.g., 0).
At t1, the current sensing signal 192 reaches the feedback signal 158 (e.g., as shown by the waveforms 203 and 204), and the comparator 150 changes the comparison signal to the logic high level. In response, the drive signal 160 changes from the logic high level to a logic low level (e.g., as shown by the waveform 202), and the power switch 106 is opened (e.g., being turned off). The current sensing signal 192 decreases rapidly to a low magnitude (e.g., 0). The signal 198 (e.g., INV) which is associated with the winding voltage 196 of the auxiliary winding 112 increases rapidly to a magnitude 210 (e.g., as shown by the waveform 206). The system 100 enters the demagnetization process.
After the completion of the demagnetization process, the series resonance occurs between the primary winding 108 and the parasitic capacitor 168 of the power switch 106. At t2, the signal 198 (e.g., INV) changes to a local minimum magnitude 212 (e.g., as shown by the waveform 206), and a pulse is generated in the detection signal 194 by the demagnetization detector 152 (e.g., as shown by the waveform 208). In response, the drive signal 160 changes from the logic low level to the logic high level (e.g., as shown by the waveform 202), and the power switch 106 is closed (e.g., being turned on) again.
The system 100 may not solve many problems related to system control in the quasi-resonant mode, such as noises. Hence it is highly desirable to improve the techniques of regulating power conversion systems operating in the quasi-resonant mode.